1. Technical Field
This disclosure relates to microelectromechanical systems (MEMS), including implantable microfluidic drug delivery systems.
2. Description of Related Art
Microfluidic drug delivery systems can be implanted in living organisms, such as in the eye of a human being. These systems may include a storage reservoir which stores a fluidic drug, an actuator which controllably expels the fluid drug from the storage reservoir, and a cannula which transfers the expelled fluid from the storage reservoir to a delivery location.
Different types of actuators have been used, including interdigitated electrochemical microelectrode pumps, microbellows, and electrolysis diaphragm actuators. These actuators may be made separately and integrated into the system. These actuators may include a diaphragm and a pump base having a pair of electrodes. When current is applied, electrolysis may take place, breaking an electrolyte into oxygen and hydrogen gases. These gases may force the diaphragm to expand. The expansion of the diaphragm, in turn, may expel fluid from the storage reservoir.
There can be difficulties with these implantable devices. During electrolysis, for example, the diaphragm may yield less-than-desirable performance (e.g., large dead volume, large stress/strain anchor points, durability, and limited expansion). The pumps may also have limited range of use or applications because they may not be scalable in size (e.g., for use in a mouse). Another concern may be with interdigitated electrodes used for the electrolysis. The associated electrolysis power efficiency (less than 50%), performance, and durability of the microelectrode may be unreliable and highly dependent on the design and fabrication process of the electrode. When the device is off and no electrolysis is taking place, there may be an uncontrolled release of fluid from the dispensing orifice. The gases may also recombine to reverse the pressure gradient and may create a suction, pulling in the fluid that had just been dispensed into the body at the orifice of the cannula. Over the life cycle of the device, the fluid reservoir may decrease in volume as fluid is dispensed. This dynamic and ever-decreasing volume of dispense fluid may make it difficult to calculate the actual fluid dispensing rate at any given time. It may also be difficult to refill and/or change the contents of the reservoir, as well as to extract a fluid sample from the living host.